Relaxed silicon-germanium (SiGe) virtual substrates, including a tensilely strained layer and a relaxed underlying layer, enable the production of novel silicon (Si)-, germanium (Ge)-, and SiGe-based devices such as field-effect transistors (FETs). A “virtual substrate” includes a layer of SiGe that has been relaxed to its equilibrium lattice constant (i.e., one that is larger than that of Si). This relaxed SiGe layer can be directly applied to a Si substrate (e.g., by wafer bonding or direct epitaxy) or atop a graded SiGe layer, in which the lattice constant of the SiGe material has been increased gradually over the thickness of the layer. The SiGe virtual substrate can also incorporate buried insulating layers, in the manner of a silicon-on-insulator (SOT) wafer. In order to fabricate high-performance devices on these platforms, thin strained layers of Si, Ge, or SiGe are grown on the relaxed SiGe virtual substrates. The resulting biaxial tensile or compressive strain alters the carrier mobilities in the layers, enabling the fabrication of high-speed and/or low-power devices. Utilizing both strain and bandgap engineering, modulation-doped FETs (MODFETs) and metal-oxide-semiconductor FETs (MOSFETs) may be tailored for enhanced performance analog or digital applications. However, because these devices are fabricated on Si/SiGe virtual substrates rather than the Si substrates commonly utilized for complementary MOS (CMOS) technologies, they present new processing challenges.
One processing challenge to device fabrication on Si/SiGe virtual substrates is the definition of dynamic random access memory (DRAM) trench storage capacitors. DRAM storage capacitors require high quality insulating layers for storing charge. A conventional DRAM trench storage capacitor formed on, for example, a p-type Si substrate may include an outer plate of, e.g., n-type doped Si substrate material (also referred to as the buried plate), a high-quality insulator grown on the outer plate, and an inner plate of, e.g., n-type doped polysilicon. Thus, two conducting plates are separated by an insulating plate. Traditionally, in DRAM trench capacitors fabricated in bulk Si substrates, the insulator is a thermally-grown silicon dioxide layer or nitrided silicon dioxide.
A conventional DRAM trench storage capacitor structure ordinarily is not suitable for fabrication on SiGe virtual substrates. The trench for this structure may be 5-10 micrometers (μm) deep, and would, therefore, extend into the relaxed SiGe layer. This layer may have many defects due to a high density of dislocations necessary to relax the strain in this layer. This high defect density, in turn, may interfere with subsequent efforts to create an insulator. In particular, thermal growth may be hampered by the defects and may result in a leaky insulator with many imperfections. Thus, the high defect density of the relaxed SiGe prevents the formation of the good-quality thermal dielectric incorporated in conventional DRAM trench capacitors. Furthermore, the nature of the insulator produced by thermal oxidation of SiGe is not well understood, so that even if the trench is etched in a defect-free SiGe region, in some applications, the thermal oxide may not be of sufficiently good quality for DRAM charge storage.
An additional challenge to the formation of DRAM trench capacitors in SiGe virtual substrates pertains to the formation of the outer plate. In conventional processes, the outer or buried plate is created for electrical isolation, either by implantation or by out-diffusion from a sacrificial dopant source deposited in the trench, prior to trench sidewall oxidation and inner-plate deposition. For either method of outer plate formation, the thermal steps used in conventional trench-capacitor processes for proper distribution of dopants may employ temperatures too high for use with Si/SiGe substrates.
Despite the inherent challenges, a DRAM trench capacitor is generally a better design choice for Si/SiGe substrates than, for example, stacked capacitor structures. First, the trench capacitor is more compatible with front-end CMOS device fabrication processes, because the alternative stacked capacitor is traditionally fabricated using chemical vapor deposition (CVD) processes at temperatures potentially high enough to degrade high-performance transistors. The trench capacitor is also more compatible with a back-end process involving many levels of metal interconnect, because it is inherently more planar than the stacked capacitor. These issues may be critical for applications which require CMOS performance on par with leading-edge microprocessors.